This invention relates generally to power line carrier (PLC) communication systems. In particular, the present invention provides a repeater amplifier circuit for boosting weak control signals on a PLC network, with noise discrimination and signal firewall protection.
Conventional power line carrier communication systems use existing alternating current (AC) power lines for conducting control signals between electronic devices attached to the power lines. Systems controlled in this manner include electronic appliances connected to electrical outlets, security alarm systems, garage door openers, lighting controls, dimmers and the like. In general, a pulse receiver is connected between the power line and each device that is to be controlled, and at least one pulse transmitter is connected to the power line. By utilizing the power line as the means for communication between the transmitters and receivers, such control systems can be installed without requiring the installation of additional wiring. Further, utilization of the power line also provides a greater physical range of control than may be available via infrared, ultrasonic or FM control systems.
Typically, the PLC communications are sent at a substantially higher carrier modulation frequency (i.e., frequencies at least two orders of magnitude higher than the power line frequencies), e.g. at 120 kHz to 200 kHz or higher as compared to conventional alternating current power distribution frequencies (e.g. 50/60 Hz or 200/400 Hz). There are substantial high frequency noise and interfering signals such as harmonics of the power signal, switching transients, etc. that interfere with the power line communications. Numerous techniques are known for operating in a noisy environment for example, in some cases, the communications messages are repeated to assure transmission, spread spectrum signals are used in other cases, in addition to many other techniques.
A major problem with power line carrier communications is spurious signals and background noise including impulse noise. Such noise can originate from the power source, the distribution network, loads coupled to the network and from remote sources. For example, the alternating current power delivered from a public utility is not a pure sine wave. The AC supply current contains harmonics that can interfere with power line communications. Additionally noise may be introduced from the loads (including switching transients). By way of illustration, if the load is a dimmer and lamp, the dimmer may “chop” the 60 Hz AC power waveform to reduce the lighting intensity. This introduces harmonics and high frequency noise on the power distribution conductors.
This makes it more difficult to communicate reliably over power lines particularly since some of the harmonics and noise associated with the power distribution fall within the frequency range of the communications signals. Such noise is not constant with respect to time, it also varies from place to place in the power distribution network. Moreover, a typical AC power line network is used for power distribution to several electrical load devices. Each of a variety of load devices can conduct a significant level of noise back onto the power line. Different loads and control devices produce different types and degrees of noise that may interfere with the flow of information over the power line.
Another problem potentially limiting a power line carrier communication system is signal attenuation. Due in part to the diverse impedance levels of the electric devices being used with a power line network, digital pulse communication signals may undergo more than 40 dB of attenuation before being captured by a receiver. This significant attenuation in combination with noise interference renders effective PLC communication very difficult.
The noise and attenuation problems existing in a particular power line network may vary substantially from one network to another depending upon on the types of devices attached to or coupled in some way to the power line network. Further, even the mode of operation of particular devices on the power line network may differentially affect the noise or attenuation levels throughout the network. For these reasons, a signaling protocol is preferred for efficiently and accurately transmitting information from a source node to a receiving node on a power line network.
One conventional signaling protocol “X-10” provides sensing, control and communications over power lines. The X-10 system was developed by Pico Electronics of Fife, Scotland and X-10 compatible products are distributed in the United States by X-10 (USA) Inc. of Northvale, N.J. The X-10 system utilizes a signaling means whereby simple control signals (i.e., on, off, dim, brighten, etc.) are transmitted over pre-existing power wires in the home for remotely controlling power to lights, appliances and the like. The X-10 power line data communication protocol is disclosed in U.S. Pat. Nos. 4,200,862; 4,628,440; and 4,638,299.
Another protocol for two-way communications links is the Electronic Industries Association Consumer Electronics Bus (CEBus) protocol for radio frequency media, power line carrier, infrared media and twisted pair media. The industry standard CEBUS (Consumer Electronics Bus) protocol (EIA IS-60) was adopted by the Electronics Industries Association (EIA). The CEBus protocol provides operating standards for establishing a local area network, or LAN, over five physical distribution media: a power line (PLBus), twisted pair (TPBus), coaxial cable (CXBus), infrared light (IRBus) and low power wireless radio (RFBus). This standard specifies how devices are to send and receive information, the media available to them for communication purposes and the format for the information the devices communicate to each other. In particular, the CEBus standard permits devices made by various manufacturers to be able to communicate with each other in a residential setting. The standard is documented in the CEBus EIA/IS-60 specification, which is fully incorporated herein by reference.
the X-10 system and as well as the CEBus and other encoded protocol systems the carrier detection threshold level is fixed. In selecting a threshold level for such a system, the level must be relatively high to provide some immunity from expected noise. The reliability of such systems is compromised when the signal-to-noise ratio is low. In particular, this increases system vulnerability to spurious signals and sensitivity to electrical noise, causing lost messages, false interpretation and spurious activation.
Different types of electrical noise that can interfere with PLC encoded protocol systems are developed on the AC power line from electronic devices such as: power tools and appliances that use induction motors, baby monitors and intercoms, electronic and magnetic ballasts, TVs, personal computers, battery charging systems and vacuum cleaners. The noise may have even originated from a neighbor who shares the same utility transformer. These types of noise disturbances can occur any time during the day, night or on a weekly basis, causing erratic communication including false turn-ON/OFF of PLC devices.
It is essential that the PLC signal strength be strong over the entire network. Low signal strength means erratic or loss of control on the most distant devices from a transmitting device, for example a remote dimmer control unit. Signal strength loss on a circuit can be due to several factors, including the number of interconnected PLC devices, power conductor losses and non-PLC devices on the circuit.
Typically, each device on the PLC network can reduce the signal strength progressively by as much as 15%–20% per device. For example: if a PLC device transmits a 4V signal strength on its power line then the next (2nd) nearest device on the same power line will load the signal strength down by 20% to 3.2V. The third device would reduce the remaining signal again by 20% to 2.6V, the fourth to 2.08V, the fifth to 1.664V, the sixth to 1.33V, the 7th to 1.065V, the 8th to 0.852V, the 9th to 0.68V, and the 10th device to 0.545V.
Power wiring losses must also be taken into account. Wiring loss is due to the power wiring resistance and capacitance losses that reduce signal strength. The greater the distance between a PLC device to another PLC device the more loss of signal strength occurs. The wiring has very little affect on signal strength if the PLC devices are in the same multi-ganged wall box or wiring distance is very short. However, for each 50 feet of 12 AWG Romex from one PLC device to another a loss of about 18% of the signal strength should be expected.
For example if a 150 ft. length of 12 AWG Romex is connected between two PLC devices and one device sends a 4.0 V signal strength command to the other unit, a signal strength of 1.77 V would be expected after losses: after the first 50 ft. of wire length, the signal strength would drop by 18% to 3.28 V, the next 50 ft. (100 ft. total) distance the signal strength would drop another 18% to 2.69 V, the last 50 ft. (150 ft. total) of distance signal strength would drop another 18% to 2.21 V. When the PLC receiving device loss of 20% included, the signal strength remaining would only be 1.12 V.
Other PLC devices or non-PLC devices on the PLC network that can degrade the signal strength include: passive PLC couplers (used to connect phases together), and non-PLC devices such as battery chargers, personal computers, laser printers and power surge suppressors. When the losses are calculated and the other factors mentioned are added in then it is easy to understand why the PLC signals may be too weak to activate a receiver. Some suppliers of PLC devices require a minimum of 50–100 mV signal strength for reliable data capture operation. Preferably, a minimum of 500 mV of signal strength should be available at any signal receiving node on the network.
Many vendors recommend using a passive coupler to couple PLC signals between phases. These couplers will further reduce the signal strength. Sometimes an amplifier repeater will be used to jump phases and amplify the signal strength. These do not solve the problem of other circuits that are on the same phase that might need to be amplified.
In a network system the PLC signal must cross phases if some PLC devices are on one phase and some PLC devices are on the other. If a passive filter (non-amplifying device) is used, a loss of about 20% of the signal strength can be expected on the other phase. These signal bridges are normally hardwired at the distribution panel. Since the distribution panel will normally be several feet from the PLC devices the wiring loses will further reduce the PLC signal strength. These loses usually cause a critical reduction of signal strength on the other phases to allow reliable communication across the PLC network.
According to conventional practice, professional installers diagnose network conflicts and determine the location of noise sources after installation. This is accomplished by the process of elimination by turning OFF circuit breakers or suspect noisy devices until the PLC devices work properly. A signal/noise analyzer may also be used to find these sources. After the noise sources have been located, they may be moved to another circuit or plug-in noise filters are used on the suspect units to prevent interference. After days, weeks or even months later another noisy device may show up and the entire PLC network must be analyzed again.